1. Field of the Invention
The present invention relates to a booster circuit for generating a higher voltage than a power supply voltage, and particularly to a charge pump style booster circuit.
2. Description of Related Art
A display unit of a portable information device such as cellular phone and Personal Digital Assistant (PDA) is configured to operate at a low power supply voltage to save power consumption. On the other hand, a display panel for displaying processed information sometimes requires a higher voltage than a power supply voltage. Generally a circuit for driving a display panel is provided with a booster circuit for boosting a power supply to generate a necessary driving voltage.
An example of booster circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2005-45934. The booster circuit is a charge pump style as shown in FIG. 3 that includes P-channel MOS transistors M1, M3 to M8, an N-channel MOS transistor M2, capacitors C1 and C2. The booster circuit is configured as shown in FIG. 3 and boosts a power supply voltage VDD by twice in response to a clock signal CLK to generate the voltage as Vout.
The circuit in FIG. 3 includes a P-channel MOS transistor M9 between a power supply line VDD and the capacitor C2 (i.e. Vout line) in order to speed up start-up at power-on.
A booster circuit needs to be operated to display information, however it is not necessary to display information at any time. Operating a booster circuit at all the time only consumes unnecessary power. Thus a boosting operation of a booster circuit is suspended while there is no information to be displayed. The circuit shown in FIG. 3 fixes a clock CLK to high-level, turns on the transistors M1 and M2, turns off the transistor M3 and M4 to charge the capacitor C1 while a boosting operation is suspended. Further, the circuit fixes a control signal CNT supplied to agate of the transistor M9 to low-level and electrically connects the VDD line and the Vout line, so that the capacitor C2 is charged to a level of VDD. Charging the capacitor C2 speeds up start-up when resuming a boosting operation.
However in recent years, there are increasing requests from clients to generate a ground level potential as Vout while a boosting operation is suspended, so as to ensure that an operation of a circuit receiving Vout is stopped while a boosting operation is suspended. Setting Vout to a ground level while a boosting operation is suspended causes a signal necessary to operate a booster circuit such as a clock signal CLK to be the ground level, thereby not ensuring to stop a boosting operation. This is because that the signal such as the clock signal CLK requires twice the VDD level during a boosting operation and is generated by a circuit operated on Vout voltage.